pwb_tests: set PYWIKIBOT2_DIR from base_dir

If the tests are run from setup.py test without PYWIKIBOT2_DIR set,
it will use base_dir='~/.pywikibot'. This is OK for all tests, except
when calling pwb.py -- which will assume base_dir is the checkout dir
instead, and thus ask for information to create a new user-config.py.

This commit makes sure PYWIKIBOT2_DIR is set to base_dir (='~/.pywikibot')
before calling pwb.py, which will make pwb.py check that directory instead.

Change-Id: I0a277425b4f9df5b64c325b7d2712f4ba007a260
Fixes-Change-Id: Ic8522e1f08c359fff4085c172e696e62ca969a70
Fixes-Change-Id: Iaeaaf1b34d3e2bcacf8308d2c3b74bd8f711ec6a
Fixes-Change-Id: If9458fca50f07f08441dbb6e06f78bdbae2065de